Methods and apparatus for separating the wavelength switching function from the channel equalization function in a reconfigurable optical add/drop multiplexer (ROADM)

ABSTRACT

In some embodiments, an apparatus includes a reconfigurable optical add-drop multiplexer (ROADM). The ROADM has a wavelength selective switch (WSS) that does not perform power equalization when the WSS is operative. The ROADM also has a first pre-amplifier, a first channel power equalizer operatively coupled to the first pre-amplifier, a second pre-amplifier operatively coupled to the first channel power equalizer and the WSS, a first post-amplifier operatively coupled to the WSS, a second channel power equalizer operatively coupled to the first post-amplifier, and a second post-amplifier operative coupled to the second channel power equalizer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/392,227, filed Dec. 28, 2016, now U.S. Pat. No. 10,341,039, andentitled “Methods and Apparatus for Separating the Wavelength SwitchingFunction from the Channel Equalization Function in a ReconfigurableOptical Add/Drop Multiplexer (ROADM)”, the disclosure of which isincorporated herein by reference in its entirety.

FIELD

One or more embodiments relate to the methods and apparatus forseparating the wavelength switching function from the channelequalization function in a reconfigurable optical add/drop multiplexer(ROADM).

BACKGROUND

The widespread implementation of communication devices has increasednetwork traffic and the need for higher bandwidth. In knowncommunication systems, for managing large, continuous and error-freedata traffic, the networks use different network topologies. Thesenetwork topologies generally include multiple interconnected nodes.Attempts are being made to improve the existing networks by developingcommunication systems that can support more complex network topologiesand greater network bandwidths.

Fiber-optic communication is typically used presently due to its largedata bandwidth and fast data transfer. One commonly implemented methodto increase data bandwidth is through Dense Wavelength DivisionMultiplexing (DWDM), which is used to multiplex data from differentoptical sources together on each optical fiber, with each optical signalhaving its own separate light wavelength. Known optical communicationsystems based on the DWDM technology typically include one or morereconfigurable optical add/drop multiplexer nodes (ROADM nodes) each ofwhich typically has multiple ROADM cards. A typical ROADM card includesa Wavelength Selective Switch (WSS) that performs both wavelengthselection and channel power equalization.

Other optical transmission systems use coherent technology for high datarate signals (e.g., 100 Gb/s & higher). Such coherent opticaltransmission systems are much less tolerant to optical signal to noiseratio (OSNR), and to impairments such as polarization dependent loss(PDL) when compared to non-coherent optical transmission systems.Coherent optical communication systems are expected to have very highport count (˜40 or more) ROADMs. But, the known ROADMs typically do notscale well beyond about 20 ports because as the number of portsincreases, at some point the optical performance of the wavelengthselective switches (WSSs) of the ROADMs degrades precipitously.

Thus, a need exists for ROADMs with higher port counts with improvedsignal-to-noise ratios and lower optical system penalties.

SUMMARY

In some embodiments, an apparatus includes a reconfigurable opticaladd-drop multiplexer (ROADM). The ROADM has a wavelength selectiveswitch (WSS) that does not perform power equalization when the WSS isoperative. The ROADM also has a first pre-amplifier, a first channelpower equalizer operatively coupled to the first pre-amplifier, a secondpre-amplifier operatively coupled to the first channel power equalizer,a first post-amplifier operatively coupled to the WSS, a second channelpower equalizer operatively coupled to the first post-amplifier, and asecond post-amplifier operative coupled to the second channel powerequalizer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a known reconfigurable optical add/dropmultiplexer (ROADM) card.

FIG. 2A illustrates an example of a ROADM card, according to anembodiment.

FIG. 2B illustrates an example of a ROADM node having multiple ROADMcards, according to an embodiment.

FIG. 2C illustrates an example of a ROADM card showing a controller,according to an embodiment.

FIG. 2D illustrates an example of a ROADM card, according to anotherembodiment.

FIG. 3 illustrates an example of a ROADM card including multipleamplification stages, according to another embodiment.

FIG. 4 illustrates a flow chart for a method implemented on the ROADMcard when the optical signal is received at Line IN, according to anembodiment.

FIG. 5 illustrates a flow chart for a method implemented on the ROADMcard when the optical signal is to be sent through Line OUT, accordingto an embodiment.

DETAILED DESCRIPTION

DWDM based-networks facilitate the transmission of optical signals viamultiple optical channels (e.g., DWDM channels) in a fiber opticalnetwork(s). Each channel is differentiated by its unique wavelength. Areconfigurable optical add-drop multiplexer (ROADM) is an element in theDWDM network, and performs various functions such as adding, dropping,passing or redirecting the optical channels of various wavelengths inthe fiber optic network(s), through the performance of wavelengthselection, channel power equalization and optical channel amplification.

Wavelength selection is typically performed by a wavelength selectionswitch (WSS) in each ROADM card of a ROADM. The WSS is used for channelselection and channel routing. For example, in FIG. 2A described below,WSS performs the task of channel routing for the received channels atthe LINE IN 142, and also performs the task of channel selection amongthe multiple channels received through the Degree (OUT) (such as 154 and158).

Channel power equalization by the ROADM is typically performed to adjustthe power levels of multiple optical channels of different wavelength inan optical beam. The channel power equalization is performed byattenuating the intensity of one or more optical channels to a uniformlevel (or to a target profile) across the various optical channels. Thechannel power equalization can be implemented, for example, by awavelength blocker, a WSS (separate from the WSS performing channelselection and channel routing) or any other appropriate device capableof performing optical channel attenuation for the channel powerequalization.

Optical channel amplification by the ROADM can be performed tostrengthen a weak optical channel(s) by using optical amplifier(s). Theweak optical channel is subjected to amplification both beforetransmission over the optical fiber network and after receiving theoptical signals that have been weakened because of its transmission viathe optical fiber network. Multiple amplifiers can be used to achievethe desired channel power, essentially, to transmit optical signals forthe optical channel through the optical fiber network with less of aneffect due to the noise. The received optical signals for the opticalchannel are subjected to amplification to enable further signalprocessing such as retransmission or reception. The optical amplifierscan be for example Erbium-Doped Fiber Amplifiers (EDFAs), Ramanamplifiers and semiconductor optical amplifiers (SOAs).

As described here, the channel power equalization and wavelengthselection functions of a ROADM are implemented using separate anddedicated hardware devices, such as channel power equalizer and WSS,respectively. Such an embodiment(s) provides the following benefits. Thechannel power equalization can be performed at a point in the ROADM thatimpacts optical system performance to a minimal extent and substantiallymaintains the signal-to-noise ratio. The WSS can be exclusivelyoptimized (or configured) for the purpose of wavelength selection. Thisallows the channel power equalizer to be separately optimized (orconfigured) for the purpose of improving the signal-to-noise ratio bymitigating the noise accumulation in the ROADM. By separating thewavelength selection function and the channel power equalization, it isfeasible to have a WSS dedicated to switching, routing, and selecting ofchannels only (without performing power equalization) and with a higherport count (˜40 or more) as compared to the WSS used in the knownsystems that has a relatively lower port count (˜20 or less). It is thusfeasible to have ROADMs with a higher port-count (˜40 or more) ascompared to the ROADMs based on the known systems that have a relativelylower port count (˜20 or less).

FIG. 1 illustrates an example of a known reconfigurable optical add/dropmultiplexer (ROADM) card 100. ROADM card 100 includes two amplifiers,pre-amplifier 110 and post-amplifier 112. Both pre-amplifier 110 andpost-amplifier 112 are physically coupled to the wavelength selectiveswitch (WSS) 130. The wavelength selective switch 130 has two functions:WSS Channel Route & Power Equalization 132 and WSS Channel Select &Power Equalization 134. WSS Channel Route & Power Equalization 132relates to the channel received at the Line IN 142; WSS Channel Select &Power Equalization 134 relates to the channel is to be transmittedthrough Line OUT 144. The channel monitor 120 receives a feedback signalfrom each of pre-amplifier 110, post-amplifier 112, WSS Channel Route &Power Equalization 132 and WSS Channel Select & Power Equalization 134.The channel(s) (or optical signals) enters the ROADM card 100 throughLine IN 142 and/or Port (IN) (such as 154 and 158). The channel(s) (oroptical signals) exits the ROADM card 100 through Line OUT 144 and/orDegree (OUT) (such as 152 and 156).

Channel monitor 120 can be, for example, a hardware device and/orsoftware (executed on a processor) capable of performing one or morefunctions of monitoring the various components of the ROADM card 100 andproviding feedback to alter the behavior of the WSS 130 of the ROADMcard 100, as described herein. The channel monitor 120 receives feedbackoptical signals from pre-amplifier 110, post-amplifier 112, WSS ChannelRoute & Power Equalization 132 and WSS Channel Select & PowerEqualization 134. Based on these feedback signals, channel monitor 120sends electrical signals to WSS 130 to alter the channel powerequalization process. When implemented as a hardware device, the channelmonitor 120 can include, for example, one or more photodetectors and/orone or more optical filters, a general purpose processor, afield-programmable gate array (FPGA), an application specific integratedcircuit (ASIC), a digital signal processor (DSP), and/or the like. Sucha processor can access memory that can store software-based instructions(or computer code) that can be executed by the processor to perform thefunctions of channel monitor 120.

Multiple ROADM cards can be included within a given ROADM node (notshown in FIG. 1). These multiple ROADM cards can be interconnected witheach other through Port (IN)/Degree (OUT) ports (such as 152 and 154,respectively) of ROADM card 100. As shown in FIG. 1, the Port 1(IN) 154is an input port for receiving at the ROADM card 100 a signal generatedat the ROADM node that includes ROADM card 100; Degree 1 (OUT) 152 is anoutput port for local routing from ROADM card 100. Additionally, for aROADM with ‘N’ cards, the ROADM card 100 will be coupled to the n^(th)ROADM card through Port (IN) 158 and Degree (Out) 156.

The WSS Channel Route & Power Equalization 132 receives input from theLine IN 142 via the pre-amplifier 110. The pre-amplifier 110 amplifiesthe received optical signals for the associated optical channel. The WSSChannel Route & Power Equalization 132 then performs channel routeselection and further sends the output to the various Degree (OUT) ports(such as 152 and 156). The WSS Channel Select & Power Equalization 134receives inputs from Port (IN) (such as 154 and 158). The WSS ChannelSelect & Power Equalization 134 then selects the appropriate opticalchannel and performs channel power equalization. Next, the opticalsignal is sent to the post-amplifier 112 for amplification and then toLine OUT 144.

The ROADM node having the ROADM card 100 is not ideal from an opticalsystem performance and noise accumulation point of view because the“channel power equalization” functionality performed by WSS ChannelRoute & Power Equalization 132 induces signal attenuation that couldotherwise be implemented post-amplification. The “wavelength selection”functionality of WSS Channel Select & Power Equalization 134 involvesassembling a DWDM signal to be amplified and transmitted (i.e. to beimplemented pre-amplification). The wavelength selection function andthe channel power equalization function have competing opticalperformance requirements in optical design, making it difficult for theknown ROADM to provide both functionalities within the WSS and at thesame time meet tighter specifications in coherent optical systems forhigh port counts (e.g., at least 40 ports) and for high data rates(e.g., 100 Gb/s & higher). Accordingly, the embodiment shown withrespect to FIG. 2A provides an alternative design that can provideimproved performance of the specifications in coherent optical systemsfor high port counts (e.g., up to 40 ports or more) and for high datarates (e.g., 100 Gb/s & higher).

FIG. 2A illustrates an example of a ROADM card 200, according to anembodiment. The ROADM card 200 includes two sets of amplifiers, a set ofpre-amplifiers (110 and 111) and a set of post-amplifiers (112 and 113).Both the pre-amplifiers (110 and 111) and the post-amplifiers (112 and113) are operatively coupled to the wavelength selective switch (WSS)230. The wavelength selective switch 230 performs WSS Channel Route 232and WSS Channel Select 234. WSS Channel Route 232 relates to the channelreceived at the Line IN 142; WSS Channel Select 234 relates to thechannel to be transmitted through Line OUT 144. The channel powerequalizer 210 performs power equalization on the optical channel inbetween the pre-amplification stages (110 and 111) of the opticalchannels. Similarly, the channel power equalizer 212 performs powerequalization on the optical channel in between the post-amplificationstages (112 and 113) of the optical channels. The channel monitor 120receives a feedback optical signal from some or all of pre-amplifiers110 and 111, post-amplifiers 112 and 113, channel power equalizers 210and 212, WSS Channel Route 232 and WSS Channel Select 234.

The channel(s) (or optical signals) enters the ROADM card 200 throughLine IN 142 and/or Ports (IN) (such as 154 through 158). The channel(s)(or optical signals) exits the ROADM card 200 through Line OUT 144and/or Degree (OUT) (such as 152 and 156). Although the FIGS. 2 and 3illustrate only two ports namely, Port 1 (IN)/Degree 1 (OUT) and Port N(IN)/Degree N (OUT), it is understood that an actual implementation caninclude multiple ports ranging from 1 to N, where N is a natural number(e.g., N can be 40). Furthermore, the description here regarding thefunctioning of the ROADM using Port 1 (IN)/Degree 1 (OUT) is alsoapplicable to the other ports for up to N ports.

ROADMs are associated with a transmission fiber pair and can bedescribed in terms of degrees of switching direction. Typically, thedegrees range from a minimum of two to as many as eight degrees, andoccasionally more than eight degrees. For example, a two-degree ROADMswitches in two directions. For another example, a four-degree ROADMswitches in four directions. In a ROADM having multiple ROADM cards 200,each degree of the ROADM involves a ROADM card 200 with its own WSS 230,and its own channel power equalizers 210 and 212. It should beunderstood that not all degrees of a ROADM have to be in use at anygiven time; rather it is possible that a ROADM can have multiple degreesbut only a subset are in use at a given time.

Multiple ROADM cards can be included within a given ROADM node (shown inFIG. 2B). These multiple ROADM cards can be interconnected with eachother through Port (IN)/Degree (Out) ports (such as 152 and 154,respectively) of ROADM card 200. As shown in FIG. 2A, the Port 1(IN) 154is an input port for receiving at the ROADM card 200 a signal generatedat another ROADM card within the ROADM node; Degree 1(OUT) 152 is anoutput port for routing from ROADM card 200 to another ROADM card withinthe ROADM node. Additionally, for a ROADM node with ‘N’ cards, the ROADMcard 200 will be coupled to the n^(th) ROADM card through Port (IN) 158and Degree (Out) 156.

The WSS Channel Route 232 receives input from the Line IN 142 via thepre-amplifiers 110 and the channel power equalizer 210. The opticalsignal(s) for each optical channel is received at the stage 1pre-amplifier 110, which amplifies the received optical signal(s) forthe associated optical channel(s). The pre-amplified optical signal(s)for each optical channel is then provided to the channel power equalizer210, which performs power equalization on the received optical signal(s)for each optical channel. Next, the optical signal(s) for each opticalchannel is sent to the stage 2 pre-amplifier 110 and then sent to theWSS Channel Route 232. The WSS Channel Route 232 then performs channelroute selection and further outputs the optical signal(s) for eachoptical channel to the various Degree (OUT) ports (such as 152 and 156).

The WSS Channel Select 234 receives input from Port (IN) (such as 154and 158). The WSS Channel Select 234 then selects the appropriateoptical channel(s) and transmits the optical signal(s) for the selectedoptical channel(s) to the stage 1 post-amplifier 112 for amplification.Next, the optical signal(s) for the selected optical channel(s) is sentto the channel power equalizer 210, which performs power equalization onthe various optical channels. The optical signal(s) for the variouspower-equalized optical channels is further sent to the stage 2post-amplifier 112 for amplification and then to Line OUT 144.

In one of the embodiment, the channel power equalizer 210 is operativelycoupled with pre-amplifiers 110 during the reception of the opticalchannel through Line IN 142. The channel power equalizer 212 isoperatively coupled with post-amplifiers 112 during the transmission ofthe optical channel through Line OUT 144. Although FIG. 2A shows channelpower equalization performed by the channel power equalizers (210 and212), it should be understood that the channel power equalizers (210 and212) can be implemented, for example as shown in FIG. 2D, by awavelength blocker, a WSS (210′ and 212′), or any other appropriatedevice capable of performing optical channel power adjustment (viaattenuation for example) for channel power equalization. In other words,the channel power equalizers can be implemented by any type ofappropriate optical device that is capable of manipulating certainspectral slides of optical power independent of other spectral slices ofthe same optical power. It is further understood that the channel powerequalizers (210 and 212) can be physically implemented in a singleoptical module or in two separate modules.

As shown in FIG. 2C, a ROADM card can include a controller 125, whichreceives a signal from the channel monitor 120. The controller 125 cansend a control signal to the first channel power equalizer 210 and theWSS 232. Alternatively or in addition, the controller 125 can send acontrol signal to the second channel power equalizer 212 and WSS 234.

FIG. 3 illustrates an example of a ROADM card 300 including multipleamplification stages, according to another embodiment. The ROADM card300 includes two amplifying stages, pre-amplifying stages (310 and 314)and post-amplifying stages (320 and 324). Both pre-amplifying stage 314and post-amplifying stage 320 are operatively coupled to the wavelengthselective switch (WSS) 230. WWS 230 performs WSS Channel Route 232 andWSS Channel Select 234. WSS Channel Route 232 relates to the channelreceived at the Line IN 142; WSS Channel Select 234 relate to thechannel is to be transmitted through Line OUT 144. The channel powerequalizers (210 and 212) perform power equalization on the opticalchannel(s) in between the amplification stages (310 & 314 and 320 & 324)of the optical channel. The channel monitor 120 receives a feedbackoptical signal from some or all of pre-amplifier stages (310 and 314),post-amplifier stages (320 and 324), channel power equalizers (210 and212), WSS Channel Route 232 and WSS Channel Select 234. The opticalsignal(s) for the associated optical channels is received at the ROADMcard 300 through Line IN 142 and/or Port (IN) (such as 154 and 158). Theoptical signal(s) for the associated optical channels exits the ROADMcard 300 through Line OUT 144 and/or Degree (OUT) (such as 152 and 156).

Multiple ROADM cards can be included within a given ROADM node (notshown in FIG. 3). These multiple ROADM cards can be interconnected witheach other through Port (IN)/Degree (Out) ports (such as 152 and 154,respectively) of ROADM card 300. As shown in FIG. 3, the Port 1(IN) 154is an input port for receiving at the ROADM card 300 a signal generatedat another ROADM card within the ROADM node; Degree 1 (OUT) 152 is anoutput port for routing from ROADM card 300 to another ROADM card withinthe ROADM node. Additionally, for a ROADM node with ‘N’ cards, the ROADMcard 300 will be coupled to the n^(th) ROADM card through Port (IN) 158and Degree (Out) 156.

The WSS Channel Route 232 receives input from the Line IN 142 via thepre-amplifiers stage 310, the channel power equalizer 210 and thepre-amplifiers stage 314. The optical signal(s) is received at thepre-amplifier stage 310, which amplifies the received optical signal(s)for the associated optical channel(s). The pre-amplified opticalsignal(s) for the associated optical channel(s) is then provided to thechannel power equalizer 210, which performs power equalization on thereceived optical signal(s) for the associated optical channel(s). Next,the optical signal(s) is sent to the pre-amplifier stage 314 and thensent to the WSS Channel Route 232. The WSS Channel Route 232 thenperforms channel route selection and further outputs the opticalsignal(s) for the associated optical channel(s) to the various Degree(OUT) ports (such as 152 and 156).

The WSS Channel Select 234 receives input from Port (IN) (such as 154and 158). The WSS Channel Select 234 then selects the appropriateoptical channel(s) and transmits the optical signal(s) for the selectedoptical channel(s) to the stage 1 post-amplifier 320 for amplification.Next, the optical signal(s) for the selected optical channel(s) is sentto the channel power equalizer 212, which performs power equalization onthe various optical channels. The optical signal(s) for the variouspower-equalized channels is further sent to the next stagepost-amplifier 324 for amplification and then to Line OUT 144.

In one of the embodiment, the channel power equalizer 210 is coupledwith pre-amplifier stage (310 and 314) during the reception of theoptical channel through Line IN 142, while the channel power equalizer212 is coupled with post-amplifier stage (320 and 324) during thetransmission of the optical channel through Line OUT 144. Although FIG.3 shows channel power equalization performed by the channel powerequalizer (210 and 212), it should be understood that the channel powerequalizer (210 and 212) can be implemented, for example, by a wavelengthblocker, a WSS or any other appropriate device capable of performingoptical channel power adjustment (via power adjustment for example) forchannel power equalization. It is further understood that the channelpower equalizers (210 and 212) can be physically implemented in a singleoptical module or in two separate modules.

In one implementation, the amplifiers in the pre-amplification stage(310 and 314) and post-amplification stage (320 and 324) can be cascadedto obtain the desired optical gain. Further, it is understood that thecascading of the optical amplifiers is design dependent and may varybased on the desired performance.

FIG. 4 illustrates a flow chart for a method implemented on the ROADMcard when the optical signal is received at Line IN , according to anembodiment. For example, FIG. 4 can be performed on optical signalsreceived at Line IN 142 of the ROADM cards 200 and 300 of FIGS. 2A and3, respectively. At 410, a first set of optical signals associated witha set of optical channels are amplified to produce a second set ofoptical signals associated with the set of optical channels. Forexample, for the ROADM card 200 shown in FIG. 2A, the optical signalsreceived at Line IN 142 can be amplified using optical pre-amplifier110.

At 420, the set of optical channels for the second set of opticalsignals are subjected to the process of channel power equalization toproduce a third set of optical signals. During this process, the powerof one or more optical signals from the second set of optical signalsare adjusted (via attenuation as an example) to a common power level (ortarget power profile). For example, for the ROADM card 200 shown in FIG.2A, the optical signals received at channel power equalizer 210 can bepower equalized to produce optical signals output from the channel powerequalizer 210.

At 430, the third set of optical signal are amplified to produce afourth set of optical signals. For example, for the ROADM card shown inFIG. 2A, the optical signals received at the pre-amplifier 111 areamplified and sent to the WSS 232.

At 440, for each optical channel from the set of optical channels, anoptical signal from the fourth set of optical signals is routed to oneor more of the output ports/degrees (such as 152 and 156) withoutperforming equalization on the fourth set of optical signals. Forexample, for the ROADM card shown in FIG. 2A, the WSS 232, for eachoptical channel from the set of optical channels, routes that channel toone or more of the output ports/degrees (such as 152 and 156). WSS 232does not perform power equalization on the routed optical signals.

FIG. 5 illustrates a flow chart for a method implemented on the ROADMcard when the optical signal is to be sent through Line OUT, accordingto an embodiment. At 510, for each optical channel (each opticalwavelength) from all sets of optical channels presented at ROADMport/degree IN (such as 154 and 158), an optical signal from a first setof optical signals is selected to produce a second set of opticalsignals; this selection is performed without performing powerequalization on the first set of optical signals. For example, for theROADM card 200 of FIG. 2A, WSS 234 selects, for each optical channelfrom a set of optical channels, an optical signal from multiple opticalsignals associated with an optical channel, without performing powerequalization on the selected optical signals.

At 520, the second set of optical signals is amplified to produce athird set of optical signals. For example, for the ROADM card 200 ofFIG. 2A, post-amplifiers 112 can receive and amplify the optical signalsreceived from WSS 234. Post amplifier 112 can then provide the amplifiedoptical signals to channel power equalizer 212.

At 530, the third set of optical signals can be subjected to powerequalization to produce a fourth set of optical signals. During thisprocess, the power of one or more optical signals from the third set ofoptical signals are adjusted (via attenuation as an example) to a commonpower level (or a target power profile). For example, for the ROADM card200 shown in FIG. 2A, the optical signals received at channel powerequalizer 212 can be power equalized to produce optical signals outputfrom the channel power equalizer 212 and provided to post-amplifier 113.

At 540, the fourth set of optical signals are amplified. For example,for the ROADM card 200 of FIG. 2A, post-amplifier 113 can receive thefourth set of optical signals from the channel power equalizer 212 andamplified the fourth set of optical signals. These amplified opticalsignals can then be sent to the network.

Some embodiments described herein relate to a computer storage productwith a non-transitory computer-readable medium (also can be referred toas a non-transitory processor-readable medium) having instructions orcomputer code thereon for performing various computer-implementedoperations. The computer-readable medium (or processor-readable medium)is non-transitory in the sense that it does not include transitorypropagating signals per se (e.g., a propagating electromagnetic wavecarrying information on a transmission medium such as space or a cable).The media and computer code (also can be referred to as code) may bethose designed and constructed for the specific purpose or purposes.Examples of non-transitory computer-readable media include, but are notlimited to: magnetic storage media such as hard disks, floppy disks, andmagnetic tape; optical storage media such as Compact Disc/Digital VideoDiscs (CD/DVDs), Compact Disc-Read Only Memories (CD-ROMs), andholographic devices; magneto-optical storage media such as opticaldisks; carrier wave signal processing modules; and hardware devices thatare specially configured to store and execute program code, such asApplication-Specific Integrated Circuits (ASICs), Programmable LogicDevices (PLDs), Read-Only Memory (ROM) and Random-Access Memory (RAM)devices. Other embodiments described herein relate to a computer programproduct, which can include, for example, the instructions and/orcomputer code discussed herein.

Examples of computer code include, but are not limited to, micro-code ormicroinstructions, machine instructions, such as produced by a compiler,code used to produce a web service, and files containing higher-levelinstructions that are executed by a computer using an interpreter. Forexample, embodiments may be implemented using imperative programminglanguages (e.g., C, Fortran, etc.), functional programming languages(Haskell, Erlang, etc.), logical programming languages (e.g., Prolog),object oriented programming languages (e.g., Java, C++, etc.) or othersuitable programming languages and/or development tools. Additionalexamples of computer code include, but are not limited to, controlsignals, encrypted code, and compressed code.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Where methods described above indicate certain eventsoccurring in certain order, the ordering of certain events may bemodified. Additionally, certain of the events may be performedconcurrently in a parallel process when possible, as well as performedsequentially as described above.

What is claimed is:
 1. An apparatus, comprising: a reconfigurable optical add-drop multiplexer (ROADM) having: a plurality of input/output ports; a wavelength selective switch (WSS) operatively coupled to the plurality of input/output ports without an amplifier between the WSS and the plurality of input/output ports, the WSS does not perform power equalization when the WSS is operative, the WSS is configured to select, for each optical channel from a plurality of optical channels, an optical signal from a first plurality of optical signals without optimizing collectively for input power disparity, insertion loss, polarization dependent loss and extinction ratio of the first plurality of optical signals, the first plurality of optical signals being associated with a first direction, the WSS is configured to select, for each optical channel from the plurality of optical channels, an optical signal from a second plurality of optical signals without optimizing collectively for input power disparity, insertion loss, polarization dependent loss and extinction ratio of the second plurality of optical signals, the second plurality of optical signals being associated with a second direction different from the first direction, the ROADM is a ROADM node having a plurality of ROADM cards including a first ROADM card, the first ROADM card including the WSS, a first pre-amplifier, a first channel power equalizer coupled to the first pre-amplifier, a second pre-amplifier coupled to the first channel power equalizer and the WSS, a first post-amplifier coupled to the WSS, a second channel power equalizer coupled to the first post-amplifier, and a second post-amplifier coupled to the second channel power equalizer.
 2. The apparatus of claim 1, wherein: each ROADM card from the plurality of ROADM cards being operatively coupled to each remaining ROADM card from the plurality of ROADM cards, each ROADM card from the plurality of ROADM cards having a plurality of degrees, a number of ROADM cards within the plurality of ROADM cards being equal to the number of degrees in the plurality of degrees.
 3. The apparatus of claim 1, wherein: the first pre-amplifier is configured to amplify the first plurality of optical signals to an amount optimal for the first channel power equalizer to equalize the first plurality of optical signals.
 4. The apparatus of claim 1, wherein: the first pre-amplifier is configured to amplify the first plurality of optical signals to an amount optimal for the first channel power equalizer to equalize the first plurality of optical signals to produce a third plurality of optical signals, the second pre-amplifier is configured to amplify the third plurality of optical signals to an amount greater than the amount for the first pre-amplifier.
 5. The apparatus of claim 1, wherein the ROADM further has a channel monitor configured to receive a portion of an optical signal produced by the first channel power equalizer and a portion of an optical signal produced by the WSS, the channel monitor configured to send a signal to a controller such that the first channel power equalizer and the WSS receive a control signal from the controller.
 6. An apparatus, comprising: a reconfigurable optical add-drop multiplexer (ROADM) having: a plurality of input/output ports; a wavelength selective switch (WSS) operatively coupled to the plurality of input/output ports without an amplifier between the WSS and the plurality of input/output ports, the WSS does not perform power equalization when the WSS is operative, the WSS is configured to select, for each optical channel from a plurality of optical channels, an optical signal from a first plurality of optical signals without optimizing collectively for input power disparity, insertion loss, polarization dependent loss and extinction ratio of the first plurality of optical signals, the first plurality of optical signals being associated with a first direction, the WSS is configured to select, for each optical channel from the plurality of optical channels, an optical signal from a second plurality of optical signals without optimizing collectively for input power disparity, insertion loss, polarization dependent loss and extinction ratio of the second plurality of optical signals, the second plurality of optical signals being associated with a second direction different from the first direction, a first pre-amplifier associated with the first plurality of optical signals and not the second plurality of optical signals, a first channel power equalizer operatively coupled to the first pre-amplifier, the first channel power equalizer being associated with the first plurality of optical signals and not the second plurality of optical signals, and a second pre-amplifier operatively coupled to the first channel power equalizer and the WSS, the second pre-amplifier being associated with the first plurality of optical signals and not the second plurality of optical signals.
 7. The apparatus of claim 6, wherein: the first pre-amplifier is configured to amplify the first plurality of optical signals to an amount optimal for the first channel power equalizer to equalize the first plurality of optical signals.
 8. The apparatus of claim 6, wherein: the first pre-amplifier is configured to amplify the first plurality of optical signals to an amount optimal for the first channel power equalizer to equalize the first plurality of optical signals to produce a third plurality of optical signals, the second pre-amplifier is configured to amplify the third plurality of optical signals to an amount greater than the amount for the first pre-amplifier.
 9. The apparatus of claim 6, wherein the ROADM further has a channel monitor configured to receive a portion of an optical signal produced by the first channel power equalizer and a portion of an optical signal produced by the WSS, the channel monitor configured to send a signal to a controller such that the first channel power equalizer and the WSS receive a control signal from the controller.
 10. An apparatus, comprising: a reconfigurable optical add-drop multiplexer (ROADM) having: a plurality of input/output ports; a wavelength selective switch (WSS) operatively coupled to the plurality of input/output ports without an amplifier between the WSS and the plurality of input/output ports, the WSS does not perform power equalization when the WSS is operative, the WSS is configured to select, for each optical channel from a plurality of optical channels, an optical signal from a first plurality of optical signals without optimizing collectively for input power disparity, insertion loss, polarization dependent loss and extinction ratio of the first plurality of optical signals, the first plurality of optical signals being associated with a first direction, the WSS is configured to select, for each optical channel from the plurality of optical channels, an optical signal from a second plurality of optical signals without optimizing collectively for input power disparity, insertion loss, polarization dependent loss and extinction ratio of the second plurality of optical signals, the second plurality of optical signals being associated with a second direction different from the first direction, a first post-amplifier operatively coupled to the WSS, the first post-amplifier being associated with the second plurality of optical signals and not the first plurality of optical signals, a second channel power equalizer operatively coupled to the first post-amplifier, the second channel power equalizer being associated with the second plurality of optical signals and not the first plurality of optical signals, and a second post-amplifier operative coupled to the second channel power equalizer, the second post-amplifier being associated with the second plurality of optical signals and not the first plurality of optical signals.
 11. The apparatus of claim 10, wherein: the first post-amplifier is configured to amplify the second plurality of optical signals to an amount optimal for the second channel power equalizer to equalize the second plurality of optical signals.
 12. The apparatus of claim 10, wherein: the ROADM further includes a first channel power equalizer operatively coupled to the first pre-amplifier, the first channel power equalizer being associated with the first plurality of optical signals and not the second plurality of optical signals, the first pre-amplifier is configured to amplify the first plurality of optical signals to an amount optimal for the first channel power equalizer to equalize the first plurality of optical signals to produce a third plurality of optical signals, the second pre-amplifier is configured to amplify the third plurality of optical signals to an amount greater than the amount for the first pre-amplifier, the first post-amplifier is configured to amplify the second plurality of optical signals to an amount optimal for the second channel power equalizer to equalize the second plurality of optical signals to produce a fourth plurality of optical signals, the second post-amplifier is configured to amplify the fourth plurality of optical signals to an amount greater than the amount for the first post-amplifier.
 13. The apparatus of claim 10, wherein the ROADM further has a channel monitor configured to receive a portion of an optical signal produced by the second channel power equalizer and a portion of an optical signal produced by the WSS, the channel monitor configured to send a signal to a controller such that the second channel power equalizer and the WSS receive a control signal from the controller. 